MPLS + BGP Advanced Features

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Will LDP assign tags to BGP routes? Why?

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Before---We said that LDP assigns tags to IGP and never says to assign a label to BGP.

Here's an experiment to illustrate how LDP+BGP works.

How to forward a pure IP network:

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In the topology diagram, R1 and R4 establish EBGP neighbors. R3 and R5 build EBGP neighbors.

Establish IBGP neighbors between R1 and R3.

The IGP protocol between R1-R2-R3 is OSPF.

R4 BGP in Network 44.1.1.1 mask 255.255.255.255

R5 BGP in Network 55.1.1.1 mask 255.255.255.255

Problem:

Eventually, R4 and R5 will learn the BGP route to the peer through BGP. First, there is a problem, is R4 ping 55.1.1.1 source loo0, or R5 ping 44.1.1.1 sou loo0, do you?

-------------answer is not a pass. It can be proved by doing experiments. The reason is R2 above. If the pure IP, R2 is a routed black hole.

Above the R2, there is neither a 44.1.1.1 route nor a 55.1.1.1 route. If you want to make R4 and R5 two routers pass, unless, let R2 also participate in BGP, the method is many. R2 as RR Route reflector. R1 and R3 are both clients of the R2. or with the alliance. In short, make sure that R2 can also learn the relevant 44.1.1.1/55.1.1.1 routes, then in a pure IP environment in order to communicate.

Otherwise R2 is a routed black hole.

MPLS Label Forwarding method:

The topology diagram is still the same as the intermediate as 100 zone, with MPLS IP running between R1,R2 and R3.

At this time, R2 still does not run BGP.

Other conditions are consistent with the above pure IP forwarding.

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In this case, R4 network 44.1.1.1/32 enters BGP. R5 network 55.1.1.1/32 into BGP.

Ask : is R4 can be Ping Pass R5 , Why ?

-------through the experiment found that can pass.

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Conclusion: above the ASBR, an IP packet was received.

For R1, the source of the message is 44.1.1.1. The purpose is to 55.1.1.1, from R1 E0/1 into IP, from R1 to e0/0 MPLS.

The destination is to go to a BGP entry. Then this time will be a tag, this time will give the BGP route to the next hop tag.

This is why R2 does not go to the 55.1.1.1 or 44.1.1.1 route, there will still be tags, because the label is for 11.1.1.1 or 33.1.1.1. The R2 has a route prefix and has associated tag mappings.

Therefore, make sure that the ASBR has detailed routing on it. Otherwise the packet is dropped to ASBR because the core routing table has no entries.

Let's take a look at how this data flow goes:

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Above the R4, ping 55.1.1.1 source loo0.

1, first R4 need to have 55.1.1.1 prefix routing information.

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Here in R4 's core routing table, for that route, the next hop is 14.1.1.1. Then the packet is sent to the R1.

2, when the IP message is sent to R1, is the IP--->mpls, the CEF table should be viewed.

The following are all related table entries for R1.

First, R1 has the route of the core routing table:

Show IP route

B 55.1.1.1 [200/0] via 33.1.1.1, 02:40:13

Here, R1 to 55.1.1.1,BGP route the next hop is 33.1.1.1.

Second, the show IP cef table, found 55.1.1.1 prefix, was assigned a label=17 by the remote R2.

Again, in Lfib, it was found that the 17 tag is actually a label assigned to 33.1.1.1 (which is exactly the BGP prefix next hop).

Finally, the packet is push into a 17 label and forwarded to R2 via R1 's e0/0.

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3, when the MPLS packet, carrying Label=17, entered the R2 e0/0.

R2 's label forwarding is as follows:

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For the incoming 17 MPLS message, the label pops up on the R2, because 33.1.1.1/32 is a direct connection to the R3, R3 assigns an implicit empty label to the 33.1.1.1 prefix and distributes it to R2. When the packet is R2, peel the label, Pop label ( PHP Countdown Second popup mechanism). The IP message is then forwarded to R3.

4, R3 received this message later. The message was found to be 55.1.1.1. Find the routing table:

R3 The above routing table:

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On R3, reach 55.1.1.1, the next hop is a direct-connect route 35.1.1.5. is up to the. So packets are forwarded to R5.

5, packet to R5 above, finally successfully forwarded.

=============================================

Think: If R1 and R3 do not use loopback as the source of BGP, and the physical interface, R4 and R5 can also communicate?

The previous application was the IBGP established by R1 and R3 with a loopback port.

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Consider whether R4 and R5 can still communicate through the MPLS domain if the IBGP neighbor is established with direct connection, why?

-----------the answer is no.

Cause: After the packet has reached R1, the routing table above the R1 becomes:

Show IP route

B 55.1.1.1 [200/0] via 23.1.1.3,

R2 would think that 23.1.1.0/24 is its own direct-attached route, then R2 assigns an empty label to 23.1.1.0/24. Distribute to R1 and R3.

R1 above for 23.1.1.0 is not labeled.

After the IP packet to the R2, R2 query, no tag, then query the routing table, because R2 is not enabled BGP, there is no 44.1.1.1/55.1.1.1 route, so R2 became a routed black hole. The main purpose is to use tags to send packets using label Exchange to R3 above, so that the R3 can be routed query forward data.

This article is from the "hank--me, Focus on Principles" blog, be sure to keep this source http://361531.blog.51cto.com/351531/1699261

MPLS + BGP Advanced Features