From: Kevin M. Woods (kev@xxxxxxx)
Date: Sat May 13 2000 - 16:10:03 GMT-3
Jack is absolutely correct here, but I do want to clarify a couple of points.
The first being that synchronization is to avoid black holes whereas AS_PATH
is to avoid routing loops (I feel this is important to understand in order to
make requirements such as full-mesh IBGP and such more intuitive).
The second being that A will not only suppress the update from C, but it will
not use the update in its best path selection algorithm either. This is why
no route shows up in the routing table as Kevin discovered (I really like his
name for some reason).
But the real question is why can't A use the route? Jack pointed out that if
a router C then forwarded traffic to A destined to B then A would discard the
traffic. The premise is correct, but really A is not worried about itself
discarding traffic (why should it, it has the all the information it needs
to forward any packets to B). Instead A is worried about other routers in
its AS that are only running an IGP discarding traffic since it sees that its
own IGP does not have the route either. In other words, it realizes that the
IGP is not yet "synchronized" so it suppresses the update until it does or
synchronization is disabled. So what does "synchronized" mean: routes are
redistributed from BGP into the IGP and and the IGP has converged.
Convergence takes time so that is what A is waiting for. If it never happens
then, well, I guess you lose points unless you disabled synchronization 8-).
Of course there are no other routers in the topology we are discussing, but A
has no real way of knowing that.
In addition to Halabi's book (second edition on its way I hear!) I would also
recommend RFC 1772, Appendix A for more information on this topic.
Kevin
// Kevin,
//
// The nightmare of each routing protocols is Route Looping, to avoid this, eve
ry routing protocol will have some methods set.
//
// According to BGP, if A learned the IBGP route from B (redistributed from RIP
), A will check its routing table to see if this route could be reached via oth
er IP routing protocols, if it couldn't, then A will not send this route to it'
s EBGP neighbor (assume it's C). Otherwise if C learned this route from A and w
ill send traffic to A, actually, A did not have route to reach the dest. The tr
affic then will be discarded.
//
// you can call me to discuss.
//
// Jack
//
// -----Original Message-----
// From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
// Kevin Young
// Sent: Saturday, May 13, 2000 7:08 PM
// To: ccielab
// Subject: BGP question
//
//
// Hi,guys, there is a question puzzled me.
// Halabi'book said synchronization within an AS is IBGP peer check for the
existence of the destination in the IGP,to dicide to whether announce it to oth
er EBGP peers,
// Does it also influence the bgp route which learn from other IBGP peer enter
into itself's route table?
// this is the phenomenon , A and B are IBGP peer, B redistributing RIP rou
tes into BGP,then B advertise the routes to A, I can see the routes in A's BGP
table, but couldn't see them in A's route table, when no synchonization in A, t
he routes appear in A'route table,
// Why? wish someone give me light ,thanks.
//
//
// **************************************
// Kevin Young
// Senior Network Engineer
// Yinxi Electronic Information Co.,Ltd
// (86)-10-82625798 x 810
// **************************************
//
//
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