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Showing posts with the label Routing

How to enable IPv6 on traditional Cisco Catalyst Switches

Traditional catalyst switches like Cisco Catalyst 3750 / 3560, by default do not have IPv6 routing enabled, and simply entering the " ipv6 unicast-routing " won't work! First, enable dual-stack routing by: sdm prefer dual-ipv4-and-ipv6 This should be followed by a reboot, post which enable IPv6 routing by: ipv6 unicast-routing Post which you should be able to do all the IPv6 configuration..

OSPF Options Field

OSPF Options Field DN = Down bit : used in MPLS Layer 3 VPNs. If a route learnt from a customer network via OSPF is advertised across a BGP / MPLS VPN using Multiprotocol BGP, and is advertised back to a customer network via OSPF, there is a possibility of a loop to occur in which the OSPF route is redistributed back to the VPN service provider network via BGP. The DN bit prevents this type of routing loop. O = The “O” bit is set when the originating router supports Type 9,10 and 11 opaque LSAs. Not used in normal OSPF implementations DC = The “DC” bit is set when the originating router supports OSPF over Demand Circuits. Not used in normal OSPF implementations L = Indicates whether the OSPF packet contains a LLS (Link-Local signalling) data block. This bit is set only in Hello and DBD packets. N = The N bit is used only in Hello packets. The N bit is set when the originating router supports Type-7 NSSA-External-LSAs. Neighboring routers with mismatched N bit value will not form

BGP Best Path Selection Algorithm

Check if the next-hop is reachable. If yes, then proceed The path which has the highest WEIGHT The path which has the highest LOCAL_PREF The path which has been locally originated via a “network” or “aggregate” BGP command or through redistribution from an IGP The path which has the shortest AS_PATH The path which has the lowest origin type The path which has the lowest MED EBGP is preferred over IBGP Prefer the path which has lowest IGP metric to the BGP next hop Determine if multiple paths require installation in the routing table for “BGP multipath” In case both the paths are external, prefer the path which was received first (the oldest one) Prefer the route which comes from the BGP router with the lowest router ID If the originator or router ID is the same for multiple paths, prefer the path which has minimum cluster list length. (This is only present in BGP RR environments. It allows clients to peer with RRs or clients in other clusters. In this scenario, the client must be awa

BGP Attributes

BGP attributes are broadly divided into two parts: Well Known attributes Optional attributes Well-known attributes : "MUST" be recognized by all BGP implementations Optional attributes : May or may not be supported by the BGP implementations Transitive attributes : BGP process has to accept the path in which it is included and should pass it on to other peers even if these attributes are not supported. Meaning if any optional attribute is not recognized by a BGP implementation, then BGP looks to check if the transitive flag is set. If the transitive flag is set then BGP implementation should accept the attribute and advertise it to its other BGP Peers. Non-transitive attributes : If the BGP process does not recognize the attribute then it can ignore the update and not advertise the path to its peers. If the transitive flag is not set then BGP implementation can quietly ignore the attribute, it does not have to accept and advertise this attribute to its other peers. Well kno

EIGRP K-values

The ultimate variables that decide whether a particular metric component should contribute towards EIGRP metric calculation or not are the EIGRP K-values. There are 5 in all, K1 to K5 with K1 (in conjunction with Bandwidth) and K3 (in conjunction with Delay) used primarily for EIGRP metric calculation. The K-values are not interface specific but router specific, meaning, the same set of K-values would be applicable to all the interfaces of a particular EIGRP router. Of course, changes can be done under "router eigrp ASN" command and not under interface. The command "show ip protocols" would display the current K-values. show ip protocols (EIGRP K-values) Let us now modify the K-values.. EIGRP metric weights The excerpt shows that the moment K values are modified, the EIGRP adjacency goes down due to the mismatch of K-values with the neighboring router. The changes are reflected again by using the command : show ip protocols

EIGRP Interview Questions - Part 2

What is Advertised Distance in EIGRP? The Advertised Distance (AD) is the distance from a given neighbor to the destination router. Advertised distance is also called Reported Distance What is Feasible Distance? The Feasible Distance (FD) is the distance from the current router to the destination router. What is a Successor route? The EIGRP learned and computed best route that is eligible to be entered into routing table. By default, this is the route with the lowest metric and which meets the feasibility condition. What is Feasible Successor? If there is an alternate route and if it meets the feasibility condition, then it can be considered as a feasible successor. Do you use subnet mask or a wildcard mask in EIGRP "network" router command? EIGRP "network" command, by default, uses a classful network to advertise the connected networks. In order to be more specific in advertising the particular subnet, a wildcard mask is used.

EIGRP Interview Questions - Part 1

Something about EIGRP.. Enhanced Interior Gateway Routing protocol It is an enhanced distance vector routing protocol based on Diffused Update Algorithm (DUAL) to calculate shortest path. Classless routing protocol Support for VLSM, route-summarization Supports unequal cost load balancing, incremental updates What parameters should match for EIGRP formation? Primary address should be used for forming EIGRP neighborship Autonomous System number K-values In case a static neighborship is configured, it must be done on both the sides What is the multicast address of EIGRP? What are the packet types used by EIGRP? Hello Acknowledgement Update Query Reply What are the basic components of EIGRP? Diffused Update Algorithm - It is used for selecting the lowest cost loop-free path for a given destination Reliable Transport Protocol - RTP is used in EIGRP for detecting packet loss and to ensure ordered delivery of packets Protocol Independent Module - Support for IP, IPX, AppleTalk, IP

OSPF Interview Questions - Part 4

How would you speed up OSPF convergence? Configure lower hello and dead interval timers Configure ISPF - incremental SPF --> The principle of ISPF is to update the affected nodes only without rebuilding the whole tree, thereby, resulting in a faster convergence and saving CPU intensive activity of processing. Configure OSPF's SPF throttling - The SPF throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay SPF calculations during network instability. Command : "timers throttle spf [spf-start] [spf-hold] [spf-max-wait] How do you modify the cost of a default route propogated into an area? Use the command "default-information originate [always] [metric value] [metric-type value] [route-map name]" where: metric value = numerical cost metric-type value = Type 1 / Type 2 etc. route-map name = name of the route-map Command to create a stub area area xx stub Command to create a totally stub area area xx stub no

OSPF Interview Questions - Part 3

What are the tables maintained by OSPF? Neighbor Table : This table stores information about all the OSPF neighbors. Can be displayed using the command : show ip ospf neighbor Database / Topology Table : This table stores link state database of all the OSPF learned routes. Can be displayed using the command : show ip ospf database Routing table : After running the SPF algorithm on all the routes in the database table, the best route is selected and promoted to the Routing table. Can be displayed using the command : show ip route ospf What are the benefits of dividing an OSPF domain into multiple areas? Decrease routing overhead per router (this especially makes sense for internal non-backbone routers i.e. routers inside a single non-backbone area Ensure speedy convergence.. less routes implies faster processing Confine network instability into single area.. the inter-area summary routes and the routes from special areas such as stub, totally stub, NSSA, totally NSSA are usually &quo

Routing Information Protocol - Basics

Dynamic routing protocol Administrative distance - 120 Metric for best path selection - Hop count (Route with lowest hop count preferred) Maximum hop count allowed - 15 Route is considered unreachable for hop count - 16 RIP versions - RIP version 1, RIP version 2, RIP-NG RIP version 1 Classful routing protocol i.e. doesn't send subnet mask information in the update messages Sends periodic updates as broadcast Destination IP address for broadcast - Doesn't support authentication of update messages RIP version 2 Classless routing protocol but does support classful Sends periodic updates as multicast Destination IP address for multicast - Supports authentication of update messages RIP-NG Classless routing protocol (of course, this is IPv6 we are talking about) Sends periodic updates as multicast Destination IPv6 address for multicast - FF02::9

Common Routing protocol AD values

The AD values of the protocols are generally different across different vendors and care must be taken to take this fact into account in case of multi-vendor environments. Cisco Directly Connected Interface = 0 Static route = 1 EIGRP summary route = 5 External Border Gateway Protocol (eBGP) = 20 Enhanced Interior Gateway Routing Protocol (EIGRP) - Internal = 90 Open Shortest Path First (OSPF) = 110 Intermediate System to Intermediate System = 115 Routing Information Protocol (RIP) = 120 Enhanced Interior Gateway Routing Protocol (EIGRP) - External = 170 Internal Border Gateway Protocol (iBGP) = 200 Juniper Directly Connected Interface = 0 Static route = 5 Intermediate System to Intermediate System - Level-1 Internal = 15 Intermediate System to Intermediate System - Level-2 Internal = 18 Routing Information Protocol (RIP) = 100 Summary Route (Aggregate route) = 130 Open Shortest Path First (OSPF) = 150 Intermediate System to Intermediate System - Level-1 External = 160 Intermediate Syst

Routing - Administrative Distance

Most routing protocols have metric structure that are not comparable with each other, in case multiple protocols are being used in a network. For eg. RIP uses hop count and OSPF uses cost for best path selection to a particular route. In order to have a common parameter which can be used for computing the best path, the concept of Administrative Distance (AD) was introduced. AD is simply a number which helps identity a particular route learned from a particular protocol. For eg, if the route is learned from Routing Information Protocol (RIP), then it would have a default AD value of 120 (unless it has been explicitly modified). Similary, a route learned from OSPF would have the AD value of 110. Rule: A route with lower Administrative Distance (AD) value would always be preferred over the one with higher AD value. In the above scenario, if the route is being learned from both RIP as well as OSPF, then the route learned via OSPF would be preferred since OSPF has a lower AD value of 110 t