![]() The following output from debug ip cef table shows that routes with next hops which cannot be immediately resolved are queued for later processing. This recursive resolution is performed only in response to routing updates, not per packet, but it does introduce significant convergence delays. In this case, this requires three lookups: ![]() In order for CEF to resolve the next hop address of, for example, 2001:db8:0:4::1, it must perform a recursive lookup for every hop in the BGP path. ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary R1's table looks like this:Ĭodes: C - Connected, L - Local, S - Static, R - RIP, B - BGP Consider the topology below, where R2 and R3 are reflecting IBGP routes between their neighbors.Īlthough end-to-end connectivity is achieved, the routing tables on R1 and R4 are needlessly complex. The full-mesh requirement can be overcome by carefully designating intermediate routers as route reflectors, but we're still left with the issue of excessive route recursion. If IBGP is used as an IGP, this presents severe scaling problems every router would have to peer with every other router on the network. The Full Mesh Requirement and Route Recursionīy default, a BGP router will not advertise a network learned from one IBGP peer to another. By default, no meaningful metric is carried by IBGP advertisements within an autonomous system (AS). ![]() In all cases, the route with the lowest metric wins.īGP, on the other hand, employs a rather complex path selection process to determine the best route for a destination. All of these are easily tuned to meet the needs of the network operators. EIGRP uses a combination of overall delay and minimum available bandwidth, whereas OSPF and IS-IS use link cost. IGPs determine the best path for a destination based on relatively simple metrics. ![]() While it may be desirable to always statically configure neighbors in any routing protocol for security reasons, dynamic neighbor discovery is a valuable feature for less hands-on network deployments. BGP, by contrast, requires the explicit configuration of all neighbors. One of the major features of all modern interior routing protocols is the ability to automatically discover and form adjacencies with neighboring routers on links with broadcast capability. Rather, it explains why IBGP is a poor choice for the role given the availability of alternative protocols. It is important to understand that this article does not assert that IBGP cannot function as an interior routing protocol. This article discusses five major weaknesses of IBGP as interior routing protocol. Although this is typically and appropriately dismissed as a bad idea, few people take the time to examine the reasoning behind the conclusion. The term "internal" leads many newbie networkers to consider the potential deployment of IBGP as an interior routing protocol. The same protocol is run in either case, with just a few tweaks to the way route advertisement is handled. A BGP adjacency can be described as either internal (between two routers in the same autonomous system) or external (between two routers in different autonomous systems).
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