docs: Configuration guide for eBGP Multihop

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>

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+# eBGP Multihop
+
+This page explains how to configure eBGP multihop feature when external
+BGP (eBGP) peers are not directly connected and multiple IP hops away.
+
+## Prerequisites
+
+Assume you finished [Getting Started](https://github.com/osrg/gobgp/blob/master/docs/sources/getting-started.md).
+
+## Contents
+
+- [Configuration](#section0)
+- [Verification](#section1)
+
+## <a name="section0"> Configuration
+
+If eBGP neighbor "10.0.0.2" is 2 hops away, you need to configure
+`[neighbors.ebgp-multihop.config]` with `multihop-ttl >= 3` in
+`[[neighbors]]` section.
+
+```toml
+[global.config]
+as = 65001
+router-id = "10.0.0.1"
+
+[[neighbors]]
+  [neighbors.config]
+    peer-as = 65002
+    neighbor-address = "10.0.0.2"
+  [neighbors.ebgp-multihop.config]
+    enabled = true
+    multihop-ttl = 3
+```
+
+**NOTE:** eBGP Multihop feature is mututally exclusive with
+[TTL Security](https://github.com/osrg/gobgp/blob/master/docs/sources/ttl-security.md).
+These features cannot be configured for the same neighbor.
+
+## <a name="section1"> Verification
+
+Without eBGP multihop configuration, the default TTL for eBGP session is 1,
+and GoBGP cannot reach the neighbor on 2 hops away.
+
+```
+$ gobgpd -f gobgpd.toml
+{"level":"info","msg":"gobgpd started","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"Topic":"Config","level":"info","msg":"Finished reading the config file","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"level":"info","msg":"Peer 10.0.0.2 is added","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"Topic":"Peer","level":"info","msg":"Add a peer configuration for:10.0.0.2","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+...(No connection)...
+```
+
+```
+$ tcpdump -i ethXX tcp -v
+tcpdump: listening on ethXX, link-type EN10MB (Ethernet), capture size 262144 bytes
+hh:mm:ss IP (tos 0x0, ttl 1, id 19110, offset 0, flags [DF], proto TCP (6), length 60)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [S], cksum 0x8382 (incorrect -> 0x540e), seq 31213082, win 29200, options [mss 1460,sackOK,TS val 2231484 ecr 0,nop,wscale 9], length 0
+hh:mm:ss IP (tos 0x0, ttl 1, id 19111, offset 0, flags [DF], proto TCP (6), length 60)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [S], cksum 0x8382 (incorrect -> 0x5314), seq 31213082, win 29200, options [mss 1460,sackOK,TS val 2231734 ecr 0,nop,wscale 9], length 0
+hh:mm:ss IP (tos 0x0, ttl 1, id 19112, offset 0, flags [DF], proto TCP (6), length 60)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [S], cksum 0x8382 (incorrect -> 0x511f), seq 31213082, win 29200, options [mss 1460,sackOK,TS val 2232235 ecr 0,nop,wscale 9], length 0
+...(snip)...
+```
+
+With eBGP multihop configuration, GoBGP will set the given TTL for eBGP
+session and successfully connect to the neighbor on 2 hops away.
+
+```
+$ gobgpd -f gobgpd.toml
+{"level":"info","msg":"gobgpd started","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"Topic":"Config","level":"info","msg":"Finished reading the config file","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"level":"info","msg":"Peer 10.0.0.2 is added","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"Topic":"Peer","level":"info","msg":"Add a peer configuration for:10.0.0.2","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+{"Key":"10.0.0.2","State":"BGP_FSM_OPENCONFIRM","Topic":"Peer","level":"info","msg":"Peer Up","time":"YYYY-MM-DDTHH:mm:ss+09:00"}
+...(snip)...
+```
+
+```
+$ tcpdump -i ethXX tcp -v
+tcpdump: listening on ethXX, link-type EN10MB (Ethernet), capture size 262144 bytes
+hh:mm:ss IP (tos 0x0, ttl 3, id 31155, offset 0, flags [DF], proto TCP (6), length 60)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [S], cksum 0x8382 (incorrect -> 0x42a8), seq 3226540591, win 29200, options [mss 1460,sackOK,TS val 3302300 ecr 0,nop,wscale 9], length 0
+hh:mm:ss IP (tos 0x0, ttl 253, id 0, offset 0, flags [DF], proto TCP (6), length 60)
+    10.0.0.2.bgp > 10.0.0.1.xxx: Flags [S.], cksum 0x5dd6 (correct), seq 2536172214, ack 3226540592, win 28960, options [mss 1460,sackOK,TS val 3302301 ecr 3302300,nop,wscale 9], length 0
+hh:mm:ss IP (tos 0x0, ttl 3, id 31156, offset 0, flags [DF], proto TCP (6), length 52)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [.], cksum 0x837a (incorrect -> 0xfd89), ack 1, win 58, options [nop,nop,TS val 3302301 ecr 3302301], length 0
+hh:mm:ss IP (tos 0x0, ttl 3, id 31157, offset 0, flags [DF], proto TCP (6), length 103)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [P.], cksum 0x83ad (incorrect -> 0xd68c), seq 1:52, ack 1, win 58, options [nop,nop,TS val 3302301 ecr 3302301], length 51: BGP
+	Open Message (1), length: 51
+	  Version 4, my AS 65001, Holdtime 90s, ID 1.1.1.1
+	  Optional parameters, length: 22
+	    Option Capabilities Advertisement (2), length: 20
+	      Route Refresh (2), length: 0
+	      Multiprotocol Extensions (1), length: 4
+		AFI IPv4 (1), SAFI Unicast (1)
+	      Multiprotocol Extensions (1), length: 4
+		AFI IPv6 (2), SAFI Unicast (1)
+	      32-Bit AS Number (65), length: 4
+		 4 Byte AS 65001
+hh:mm:ss IP (tos 0x0, ttl 1, id 35114, offset 0, flags [DF], proto TCP (6), length 52)
+    10.0.0.2.bgp > 10.0.0.1.xxx: Flags [.], cksum 0xfd57 (correct), ack 52, win 57, options [nop,nop,TS val 3302301 ecr 3302301], length 0
+hh:mm:ss IP (tos 0x0, ttl 1, id 35115, offset 0, flags [DF], proto TCP (6), length 103)
+    10.0.0.2.bgp > 10.0.0.1.xxx: Flags [P.], cksum 0xd357 (correct), seq 1:52, ack 52, win 57, options [nop,nop,TS val 3302301 ecr 3302301], length 51: BGP
+	Open Message (1), length: 51
+	  Version 4, my AS 65002, Holdtime 90s, ID 2.2.2.2
+	  Optional parameters, length: 22
+	    Option Capabilities Advertisement (2), length: 20
+	      Route Refresh (2), length: 0
+	      Multiprotocol Extensions (1), length: 4
+		AFI IPv4 (1), SAFI Unicast (1)
+	      Multiprotocol Extensions (1), length: 4
+		AFI IPv6 (2), SAFI Unicast (1)
+	      32-Bit AS Number (65), length: 4
+		 4 Byte AS 65002
+hh:mm:ss IP (tos 0x0, ttl 3, id 31158, offset 0, flags [DF], proto TCP (6), length 52)
+    10.0.0.1.xxx > 10.0.0.2.bgp: Flags [.], cksum 0x837a (incorrect -> 0xfd23), ack 52, win 58, options [nop,nop,TS val 3302301 ecr 3302301], length 0
+hh:mm:ss IP (tos 0x0, ttl 1, id 35117, offset 0, flags [DF], proto TCP (6), length 52)
+    10.0.0.2.bgp > 10.0.0.1.xxx: Flags [.], cksum 0x837a (incorrect -> 0xfcf4), ack 71, win 57, options [nop,nop,TS val 3302311 ecr 3302301], length 0
+...(snip)...
+```